1. Field of the Invention
The present invention relates to an image forming apparatus having an image carrier, such as a photosensitive drum, or the like.
2. Description of the Related Art
An electrophotographic image forming apparatus, such as a color copier using color toners of a plurality of colors, has a lookup table for converting an image signal into a signal value suitable for the engine characteristics, so as to obtain desired tone characteristics. In a color copier, this lookup table is provided for each color of yellow, magenta, cyan, and black, and control is done to optimize the color component of each color so as to be able to output a desired full-color image. In an electrophotographic system, however, even when image forming conditions do not change, its characteristics easily change depending on the ambient circumstances, use situation, or the like. Therefore, it is difficult to continuously output images with a stable tint. To solve this problem, a technique is available which detects the density of a toner image formed on a transfer material, such as a printing sheet, or an image carrier, such as a photosensitive drum, and controls image forming conditions based on the obtained information, so as to obtain desired tone characteristics. For example, Japanese Patent Laid-Open Nos. 2005-157100 (reference 1) and 2003-337455 (reference 2) disclose a technique of correcting a lookup table and a technique of changing the charge condition or development condition of a photosensitive drum on which an electrostatic latent image is formed.
In a conventional image forming apparatus, however, since patches to be formed on an image carrier, such as a photosensitive drum, are uniquely determined, a patch for a stable toner color, which need not be corrected, is also formed, consuming toner excessively. In addition, calibration requires extra time.